The present invention relates to a process for efficiently producing a polymer sheet superior in surface smoothness and small in retardation, as well as to an optical polymer sheet produced by the process.
Glass substrates have been used as a transparent electrode substrate for liquid crystal display device. Liquid crystal display devices using a glass substrate have had problems in that the large thickness of glass substrate makes it difficult to produce a liquid crystal display device of small thickness and light weight and the use of glass incurs insufficient impact resistance.
To alleviate these drawbacks of liquid crystal display devices using a glass substrate, it has been investigated to use an optical polymer sheet for production of a liquid crystal display device of lighter weight and improved impact resistance.
For example, in JP-A-53-68099 and JP-A-54-126559, it is disclosed to continuously produce a liquid crystal display device using, in place of a glass substrate, a long polyester film having a conductive metal oxide vapor-deposited thereon. However, the polyester film (polymer sheet) has no sufficient surface smoothness, unlike a glass substrate having a very good surface smoothness as a result of polishing. When, in particular, a STN (super twisted nematic) type liquid crystal display device is produced in order to obtain a very fine display, the surface smoothness of the above polymer sheet becomes a very serious problem because in the liquid crystal display device, display is made utilizing the birefringence of the liquid crystal present between substrates having a substrate-to-substrate distance controlled at a 0.1 xcexcm accuracy.
To improve the surface smoothness of polymer sheet, a method was proposed which comprises casting, on a polished glass or the like, an ultraviolet-curing resin composition or a thermosetting resin composition, both of which are liquid, and curing the composition to obtain a sheet. This method, however, has a problem in that the sheet obtained is fragile and causes cracking or chipping during its handling; therefore, there is no clear advantage by use of the polymer sheet over use of a glass substrate. Further, the method is low in productivity, resulting in a substrate of high cost.
The present invention aims at providing a process for continuously and efficiently producing a polymer sheet superior in surface smoothness and showing excellent properties when used as an optical polymer sheet for a substrate of a liquid crystal display device or the like; and an optical polymer sheet superior in the above properties, produced using a resin suitable for the above process.
The present invention provides the followings.
(1) A process for producing a polymer sheet, which comprises coating or laminating, on a polymer base sheet, an ultraviolet-curing resin composition, adhering the coated or laminated polymer base sheet to a member having a smooth surface whose maximum surface roughness (Rmax) satisfies Rmaxxe2x89xa60.1 xcexcm, in a state that the ultraviolet-curing resin composition is soft, and applying an ultraviolet light to transfer smoothness of the smooth surface of the member onto the polymer base sheet.
(2) A process according to the above (1), wherein the member having a smooth surface can transmit an ultraviolet light of a particular wavelength and, after the ultraviolet light has been applied to the ultraviolet-curing resin composition from a side of the member not adhering to the polymer base sheet and the composition has been cured, can be peeled from the polymer base sheet.
(3) A process according to the above (1), wherein the polymer base sheet is transparent and can transmit an ultraviolet light and, after the ultraviolet light has been applied to the ultraviolet-curing resin composition from a side of the polymer base sheet not adhering to the member having a smooth surface and the composition has been cured, can be peeled from the member.
(4) A process according to any of the above (1) to (3), wherein a releasing agent is continuously fed to the member having a smooth surface.
(5) A process according to the above (4), which comprises, after application of an ultraviolet light, a step of removing the releasing agent transferred onto the surface of the polymer base sheet.
(6) A process according to the above (4) or (5), wherein the releasing agent contains at least a fluorine compound or a silicon compound.
(7) A process according to any of the above (1) to (6), wherein the member having a smooth surface is a roll.
(8) A process according to any of the above (1) to (6), wherein the member having a smooth surface is an endless belt having a flat area of 30 cm or more extending in the movement direction.
(9) A process according to any of the above (1) to (6), wherein the member having a smooth surface is a plastic film having a smooth surface.
(10) A process according to the above (9), wherein the plastic film is subjected to a slidability-imparting treatment at a side opposite to the smooth surface.
(11) An apparatus for producing a polymer sheets comprising:
a hollow transparent roll which has a smooth surface whose maximum surface roughness (Rmax) satisfies Rmaxxe2x89xa60.1 xcexcm and which can transmit an ultraviolet light, and
an ultraviolet source placed in the hollow portion of the roll.
(12) An optical polymer sheet produced by a process set forth in any of the above (1) to (10).
(13) An optical polymer sheet according to the above (12), wherein the polymer is a polyethersulfone.